Dynamics and operation of renewable-based power systems

 to loss of generation, power output of these DGs would be needed for the system to help stabilize. For this reason, new generation inverters, i.e., Smart Inverters (SIs), are required to stay connected to the system [With their advanced capabilities, they provide voltage and frequency support to help stabilize the system

Flexibility needs are divided into four categories; flexibility for power, energy, transfer capacity, and voltage . A).Flexibility for power: The power supply-demand balance is needed to maintain frequency stability for short-term periods (a second to an hour). It is due to an intermittent weather condition-dependent power supply in generation B). Flexibility for energy: The energy supply-demand balance is needed for demand scenarios over medium to long term periods(hours to several years). It is due to a decrease in fuel storage-based energy supply in generation. C). Flexibility for transfer capacity: Power transferability is needed to prevent bottlenecks over short to medium term periods (minutes to several hours). It is due to increased peak demands, increased peak supply, and increased usage levels. D). Flexibility for voltage: The bus voltages need to be kept within predefined limits over short term periods (seconds to tens of minutes). It is due to increased distributed generation in distribution system resulting in bi-directional power flow and operation scenarios variance.

the sources of power system flexibility are divided into four categories:Potential flexibility sources; are physically available and usable flexibility sources. However, they are not controllable or observable.:Actual sources of flexibility; are the usable part of potential flexibility sources because they are controllable and observable.

Flexibility reserves; are the economically usable part of the actual flexibility sources.:Flexibility reserves in the power market; are parts of the flexibility reserves that can be obtained from the power or ancillary services market

Fig. 1. Examples of flexibility solutions for each category with implementation levels from local to system wide POWER SYSTEM STABILITY